The Weeks Before Denver's First Freeze Are When Mice Move In
Denver Metro & the Front Range
Most people find out they have mice in November. The scratching in the ceiling starts on a cold night, there are droppings behind the pantry cereal, and the dog won't stop staring at the bottom of the fridge.
By then the mice have been inside for weeks. The time to deal with it is now, before the cold arrives.
You have a deadline, and it's closer than it feels
Denver's first freeze usually comes in early to mid October. The National Weather Service puts the average first freeze at October 7. Mice don't wait for a thermometer. As nights cool through late September they start looking for somewhere warmer to spend the winter, and a house with a gap under the garage door is the easiest option on the block.
That's why repairs done in September prevent the problem, and the same repairs done in December are cleanup. The hole is the same. What's different is whether mice are already living on the inside of it.
A mouse only needs a gap about a quarter inch wide
If the mouse's head fits, the body follows. A quarter inch is smaller than most people picture, about the width of a pencil, so the gaps that matter are easy to walk past for years. On Front Range homes the usual ones are:
- Garage doors. A worn or torn bottom seal is the most common way in, and new construction isn't immune. Seals get damaged in the first few winters.
- Older foundations. Many older Denver homes sit on rubble or stone foundations with dozens of small gaps where mortar has worn away.
- Utility penetrations. Where gas lines, water lines, AC lines and cable enter the house, there's often a gap around the pipe that was never sealed properly.
- Dryer vents. A vent without a working flap is an open, heated tunnel.
- Attic and soffit vents. Mice climb better than people expect, especially up brick, stucco and downspouts.
Why trapping on its own doesn't solve it
Trapping the mice already inside is necessary. But if that's all that happens, the entry point stays open, and next fall a new set of mice finds it. You end up paying for the same problem every year.
The lasting fix is two parts: remove the mice that are already in, and close the way they got in. That's why our visits include looking for entry points, not just setting traps.
Why poison isn't where we start
Store-bought poison seems like the easy answer. Here's why we don't reach for it first in a home:
- A poisoned mouse doesn't die right away. It can take days, and it often dies inside a wall. The smell that follows can last for weeks.
- Pets and kids. Loose bait gets carried, dropped and found.
- The animals that eat mice. A slow, poisoned mouse is easy prey. Owls, hawks, foxes and neighborhood cats that eat it can be poisoned too. The National Pesticide Information Center lists this "secondary poisoning" as a known risk of rodent bait.
When bait is the right tool, it goes in locked, tamper-resistant stations, usually placed outside around the home, where pets and children can't reach it. In a home with pets or kids, trapping plus a repair estimate is our starting point.
Signs they're already inside
- Small, dark, rod-shaped droppings along walls, in the pantry, or near pet food
- Scratching or light pattering in the walls, ceiling or attic at night
- Chewed cardboard, food packaging or bags of pet food and bird seed
- Shredded paper, insulation or fabric tucked into a corner or drawer
- Greasy smudges along baseboards where they run the same path over and over
- A stale, ammonia-like smell in a closed garage, cabinet or storage room
Chewed wiring is the one to take seriously. Mice gnaw constantly, and wire insulation in an attic or wall is a fire risk nobody sees until it's too late.
How we handle mice
- Active trapping. Traps go along walls and the paths mice actually use, not in the middle of the floor. Mice are wary of new objects, which is a big reason store-bought traps set out in the open catch nothing.
- Entry-point repair estimate. At the time of service our exclusion experts will inspect and give you an estimate for any repairs (exclusion) needed to keep mice from getting back in. That typically covers gaps around utility lines, garage door seals, dryer vents and foundation gaps.
- Sanitation walk-through. We walk the home with you to find what's feeding them, like pet food in bags, bird seed in the garage, or a woodpile against the house.
- Optional quarterly monitoring. For homes backing onto open space, greenbelts or empty lots, follow-up visits catch new mice before they turn into another infestation.
Things you can do this weekend
- Stand inside the closed garage in daylight. If you can see light under the door or around its edges, a mouse can get through
- Move pet food, bird seed and grass seed into metal or hard plastic bins with lids
- Stack firewood away from the house and off the ground
- Cut back shrubs and branches so they aren't touching the siding or roof
- Check that the dryer vent flap closes on its own
Frequently Asked Questions
When do mice come inside in Denver?
Mostly in late September through October, as nights cool ahead of the first freeze. Denver's first freeze usually comes in early to mid October.
How small a hole can a mouse get through?
About a quarter inch, roughly the width of a pencil. If the head fits, the body follows.
Is poison the best way to get rid of mice?
Not as a first step inside a home. Poisoned mice often die inside walls, and bait can harm pets and the wildlife that eat mice. When we use bait, it goes in locked, tamper-resistant stations.
Will mice come back after they're trapped?
If the entry points stay open, usually yes. At the time of service we inspect and give you an estimate for any repairs needed to keep them out.
Looking for more? Read about rodent control or browse the full pest library.
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